Etching of capacitor armatures



Patented Dec. 14, 1943 umrso sp res PATENT orr ctz.

York

, No Drawing. Application January 8, 1938, Serial 280.183.131

The present invention relates to" the preparation of armatures for electric capacitors.

It is knownthat animprovement may be effected in electrolytic capacitors by etching the armature size.

Heretofore, it has been the practice to chemically etch' the surface of capacitor metal by means of exposure to a solution ofacid. When aluminum is subjected to such chemical etching, careful control is required of the various etching conditions, such for example, as acid concentration, temperature of reaction, and accumulation of by-products. When the material etched by such chemical process consists of aluminum foil, it is found that foil obtained from different sources reacts diflerently with a given acid etcing bath. It is difllcult to exercise this required close contra in ordinary factory production. Even when the etching process is carried out under the supervision -of highly skilled attendants, the tests and inspection which are required slows up the rate of production.

When it is'attempted to replace the purely chemical etching procedure by electrolytic etching, the temperature, acidity and other conditions being so chosen that the removal of metal by ordinary chemical solution is substantially prevented, then deep cuts or scoring are produced in the surface of the metal being etched. As the metal armature stock commonly consists of thin foil, such cutting effect spoils or destroys the foil. gurthermore, it is dimcult to completely remove e met I have discovered that unexpectedly advantageous results are produced by 'etching armature metal by a combination of chemical and electrolytic action, the metal foil, or other armature material functioning as anode inan electrolytic bath which is capable also of producing ordinary chemicahetching of metal at a slow rate. This combination effect produces an easily controlled shallow etching and not a deep scoring of themetal. When specifying aluminur'n chloride as an accelerator of acidactivity in 'a. claim for this invention I wish also to cover equivalent chlorides such as the chlorides of sodium and copper.

While the composition'of the electrolytic bath and the conditions of electrolysis may be varied and should be governed by the. character of the metal to be etched and other conditionath'e foletching electrolyte from the deep cuts in the a 6 Claims. 00!. 204-141) V v lowing illustrative examples will illustrate my invention.

The etching electrolyte may consist of an aqueous solution of hydrochloric acid, an acid concentration well suited for etching purposes being about five per cent. agents maybe used as, for example, ammonium hydroxide, hydrofluoric acid, boric acid; sulphuric acid or acetic acid. However, hydrochloric acid is preferred. The metal to be etched, for example, aluminum foil, is caused to function as anode in such bath, the cathode conveniently consisting of carbon or other suitable inert conductive material. A bath temperature of 50 to C. is suitable. A direct current having a density of about one-quarter ampere per square inch of active foil-arca and a voltage of about 4 to 6 volts may be impressed between the foil anode and the cathodein the electrolyte. Foil to be etched may be passed through the bath at such rate that a given area functions as anode in I I the electrolytic bath for two or three minutes.

When hydrochloric acid solution is used as the etching electrolyte, its activity is increased by the presence of about two to twelve per cent of metallic chlorides as, for example, sodium chloride, aluminum chloride and copper chloride. The content of aluminum chloride preferably should be maintained as low as about 6 per cent. when other acids are used for the etching electrolyte low voltage anodic treatment to produce a light oxide film in order to prevent uncontrolled atmospheric oxidation which resultsin erratic capacity and power factor characteristic when the etched and oxidized armatures eventually are fabricated into capacitor units.

As a result of this conjoint electrolytic and nonelectroly'tic etching the rate of production of etched foil is materially increased, the consumption of acid per unit weight of metal acted upon is decreased and the uniformity of capacitors made from such etched armature foil is improved. In addition to such improvements the capacitors Other electrolytic made from, such anodically etched foil possess a more stable capacity during periods of non-use,

or as commonly stated during the shelf life" period. The non-uniformities formerly noted in capacitors containing chemically etched armature foil obtained from different sources largely are eliminamd.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of treating aluminum capacitor foil preparatory to producing a current-blocking film thereon which consists in immersing said foil in an aqueousreagent containing about 5 percent hydrochloric acid and about 2 to 12 per cent or,

aluminum chloride, maintaining said reagent at a temperature of about 50 to 75 0., conducting between said foil functioning as anode and a cooperating cathode of carbon 9. direct current at a potential of about 4 to 6 volts and a density of about one-quarter ampere per square inchof active electrode surface, thereby causing etching by a combined chemical and electrolytic action,

continuing said etching until the capacity value of the etched surface is eflectively increased, and thereupon freeing said foil from said reagent.

2. The method of treating aluminum foil preparatory to producing a current-blocking film ,thereon for use in electrolytic capacitors which consists in introducing said foil into an electrolytic bath comprising an aqueous solution containing about 5 per centof hydrochloric acidwhich is capable of producing chemical etching at a slow rate and approximately two to twelve per cent of aluminum chloride, impressing be tween said foil functioning as anode in said reagent and a cooperating cathode a direct current having at the active anode surface a density of about one-fourth ampere per square inch to increase the rate of etching, continuing such treatment for about two or three minutes for a given area to produce shallow etching without film thereon which consists in introducing said deep scoring of the etched metal surface and continuing said treatment for a length of time sufficient to cause the resulting etching of said foil to increase the efiective capacity value of theetched surface and thereupon freeing said foil from said reagent.

4. The method of treating aluminum foil preparatory to the formation of a current blocking foil into an etching electrolyte; consisting essentially of an aqueous solution of hydrochloric acid of about 5 per cent concentration and about 6 per cent of aluminum chloride, subjecting said foil to the anodic etching efiect of an electric current between said foil and a cooperating electrode in said electrolyte thereby supplementing the chemical etching eflect of said electrolyte,

continuing said treatment for a length of time sufiicient to cause the resulting etching of said foil to increase the efiective capacity value of the etched surface and thereupon freeing said foil from said electrolyte.

5." The method of treating aluminum foil preparatory to the formation of a current blocking film thereon which consists in introducing said foil into an etching electrolyte consisting essentially of an aqueous solution of hydrochloric acid of about 5 per cent concentration and about 2 to 6 per cent aluminum chloride, subjecting said foil to the anodic etching efiect of an electric current between said foil and a cooperating electrode in said electrolyte thereby supplementing the chemical etching sheet of said electrolyte, continuing said treatment for a length of time sumcient to cause the resulting etching ofsaid foil to increase the efiective capacity value of the etched surface and thereupon freeing said foil from said electrolyte. v

6. The method of treating aluminum foil preparatory to the formation of a current blocking film thereon which consists in introducing said foil into an etching. electrolyte consisting essentially of an aqueous solution of hydrochloric acid of about 5 per cent concentration and about 2 to- 12 per cent of aluminum chloride, subjecting said foil to the anodic etching efiect of an electric current between said foil and a cooperating electrode in said electrolyte thereby supplementing the chemical etching effect of said electrolyte,

continuing said treatment for a length of time su fiicient to cause the resulting etching. of said foil to increase the efliective capacity value of the etched surface and thereupon freeing said foil 5 from said electrolyte. 5

FRANK M CLARK. 

